The two spacecraft of NASA's Deep Impact
mission, dubbed Flyby and Impactor by their makers, are set to
launch Wednesday atop a Boeing Delta 2 rocket, their mission: To unlock the inner secrets of
comets.
.

"All I can do now is worry
and hope," said Deep impact principal investigator Michael A'Hearn, of the
University of Maryland, during a telephone interview. "And then watch it
go."

Deep Impact
is currently scheduled for a 1:47 p.m. EST (1847 GMT) liftoff
from Launch Pad 17B at Cape Canaveral Air Force Station in
Florida. If all goes well, the mission's two spacecraft will tag team Comet
Tempel 1 on July 4, with Impactor set to slam into the icy wanderer while
Flyby looks on.

Built for NASA by Ball
Aerospace and Technologies Corp., Deep Impact is designed to give researchers
their first glimpse of the inner workings of a comet. By crashing Impactor into
Tempel 1, thought to be a rather typical example of comets, researchers hope to
glimpse pristine material that have not changed since the formation of the solar
system.

"The interesting part of
this mission is that we don't really know what to expect," said Don Yeomans, a
senior research scientist with the Deep Impact mission at NASA's Jet Propulsion
Laboratory (JPL). "But no matter what happens, we'll observe the
phenomena."

Digging into a
comet

"We have a tremendous
amount of Earth-based data on the composition of gases that come out of a comet
and the dust lifted off its surface," A'Hearn said, adding that the object's inner
make up is still a mystery. "But the [comet] interior is characteristic of what
the solar system formed from."

To
measure that material, Deep Impact's Impactor probe -- an 820-pound
(372-kilogram), camera-equipped chunk of copper -- will be placed
in Tempel 1's path and ultimately slam into the comet at about 23,000 miles
an hour (37,014 kilometers an hour). Snapping images until the last,
Impactor is designed to give researchers their closest look yet at a
comet's surface.

Meanwhile, Flyby will record
the event -- along with a myriad of Earth-based and orbital instruments -- with
telescopes capable of two-meter resolution and an infrared spectrometer to
determine the mineral make-up of Tempel 1's
innards.

Flyby
should relay at least some data from its own sensors and Impactor's camera
during the event, but the spacecraft will continue to send observations home
until well after the initial impact.

"I really want to see the data as they come down," A'Hearn said, adding that Deep
Impact will allow researchers to look anew at previous comet observations. "On
the Fourth of July, we'll all be huddled around TV screens at mission
control."

Understanding
impacts

Comet science aside, Deep Impact is also poised to give researchers a ringside seat
to what they hope will be a huge cometary smash-up.

When
Deep Impact crashes into Comet Tempel 1, the size of its crater remains will
depend on the comet's structure and density. Size estimates from mission
scientists describe the crater as ranging from a meager 10 meters to the size of
the Rose Bowl football stadium in Pasadena, California.

"My
personal estimate [for the crater] is at the large end of a large-size football
stadium, perhaps 150 meters in diameter," A'Hearn said. "It could be
larger."

Researchers are eager to compare
Deep Impact's crater - granted the spacecraft reaches Tempel 1 successfully in
the first place - because of the past comet craters
observations.

Based on images taken by
NASA's Stardust spacecraft, which swung by the admittedly odd comet Wild
2, some astronomers believe that comet-based craters differ from their
planetary and moon-based cousins.

"They don't look like
craters anywhere else in the solar system," A'Hearn said of Wild 2's surface
features. "It's the lack of ejecta around them that makes them
unusual."

A'Hearn wants to see if
Deep Impact's crater has the same attributes, adding that the mission also has
the benefit of creating at least one crater on Tempel 1 that is
unambiguously an impact feature.

A watery
story

Some
researchers are also interested in learning the amount of water in Tempel 1,
especially since some scientists believe comets may have been the original
vehicles for water to reach Earth.

"Comets are far more than
an intellectual interest," Yeomans said, adding if comets were Earth's water
delivery system, they form a crucial cog in life's beginnings. "These things
affect us, and may have even enabled us [to exist]."

And while they may have
played a role in our beginnings, comets could one day be tapped by future human
space explorers as a source for water, oxygen and hydrogen fuel for far-flung
space missions, NASA researchers said.

"They could be the watering
holes and fuel stations for future interplanetary exploration," Yeomans
said.